This invention relates to an apparatus for producing metal powder, in which molten metal is formed into metal powder by a fluid atomizing method.
In the production of high alloy powder among metal powders used in the powder metallurgy field, an atomizing method is proposed, with such method being broadly classified into a liquid atomizing method using a liquid, composed mainly of water, as an atomizing medium and a gas atomizing method using inert gas as an atomizing medium. With this method, a large amount of metal powder having the average particle size of several .mu.m to several tens of .mu.m can be produced.
However, in order to consistently produce fine powder, it has been desired to provide technical improvements in the atomizing apparatus and the atomizing condition.
One problem which must be solved in order to consistently produce fine powder is a blocking phenomenon. Various causes for this phenomenon have been studied, and main causes with respect to an atomizing apparatus of the general type shown in FIG. 4 will now be described. Referring to one main cause, downwardly-inclined high-speed jets J are arranged to intersect each other, and are disposed symmetrically with respect to a vertical line. When molten metal M is caused to flow in a downward direction (as designated at M') toward the intersection point P2, part of the molten metal M is blown up near the intersection point (atomizing point) P2 by the high-speed jet J impinging on this molten metal M, so that part of the downward flow M' deposits on an inner surface 4 of an injection nozzle 3 to form a blockage. This tendency is apparent particularly with the gas atomizing method.
Another cause of the blocking phenomenon is an atomizing phenomenon due to a flow of the atmosphere. During the downward flow of the molten metal M to the atomizing point P2 where the molten metal M is crushed and cooled by the high-speed jet J, part of the downward flow M' is primarily atomized into molten metal fractions under the influence of the atmosphere flow, blown into the high-speed jet, or a negative pressure. Namely, this primarily-atomized fraction deposits on the inner surface 4 of the nozzle 3 to form a blockage. This tendency is apparent particularly with the liquid atomizing method.
The columnar molten metal flow M', before being crushed by the high-speed jet J, is divided by the above-mentioned primary atomizing effect, and then is vigorously crushed by the atomizing effect of the high-speed jet, and is cooled into metal powder. In the present invention, the initial division of the molten metal is referred to as "primary atomization", and the point at which this primary atomization occurs is referred to as "primary atomizing point", and the crush at the intersection point of the high-speed jet is referred to as "secondary atomization".
The above blocking phenomenon finally makes it impossible to continue the atomization. To overcome this, various proposals have been made and, for example, Japanese Patent Unexamined Publication No. 57-47805 discloses a method in which gas, introduced by an inducing effect of a high-speed water jet, is maintained by a flow straightener into a laminar flow along a downward flow of molten metal, thereby suppressing a primary atomizing effect to prevent the blockage.
In order to meet recent demands for a particle size of fine powder not to exceed 10 .mu.m, in view of sintering properties and so on, in the water atomizing method, an ultra-high pressure atomizing technique utilizing a high-speed jet with an injection pressure of about 100 MPa has now been mainly used.
With respect to the production technique disclosed in the above Japanese Patent Unexamined Publication No. 57-47805, in an ultra-high pressure atomizing method using a pressure of more than 70 MPa, it has been found that the negative pressure atmosphere due to the water jet is excessive, so that it becomes difficult to maintain the laminar flow along the downward flow of the molten metal, and therefore the blocking occurs.
In conventional fluid atomizing methods other than that disclosed in the above Japanese Publication, a negative pressure atmosphere produced by an ultra-high pressure jet is excessive, it is necessary to prevent a blocking which is caused by the deposition of primarily-atomized fractions on a surface of an injection nozzle.
Further, in order to produce a large amount of fine powder having the average particle size not exceeding 10 .mu.m, it has been proposed to adopt an ultra-high pressure atomizing method utilizing an injection jet pressure of 100 MPa or more, as described above. This serves to increase the kinetic energy of the high-speed jet to be injected, but results in a disadvantageous blocking phenomenon caused by the ultra-high pressure jet injection, as described above.